1. Field of the Invention
This invention relates to a bearing unit and more particularly to a new and novel bearing unit for use in supporting a water-lubricated propeller shaft as in large naval ships.
2. Description of the Prior Art
Bearing assembly with elastomeric bearing elements are particularly suited for this purpose because of their excellent ability to withstand the effects of corrosive fluids and to abrasion resulting from particulars of foreign matter carried in suspension in the sea water in which the shaft and bearing assembly operates. Such bearing assembly with their elastomeric bearing elements have been made and are still being made with outer non-corrosive support or shell-with a plurality of circumferentially evenly spaced elastomeric staves therein.
The science of lubricated bearings commenced in the 1880's with the discovery of considerable fluid pressure in the oil-film of bearings, from which oil-lubricated bearings were developed. In the 1890's tilting-pad oil-lubricated metal bearings were invented and later tilting-pad journal bearings were developed. In lilting-pad bearings, the pads physically move (rather than deflect) because of the very high modules or stiffness of the metal, to give the right approach angles required to develop very low hydrodynamic (self pumping) lubrication.
The term “elastohydrodynamic” evolved to cover those dynamic cases (like gear teeth) where the bearing surfaces were rapidly moving in and out of contact yet were apparently hydrodynamically lubricated, if only for periodic, very short time intervals. It was later theorized that the low friction of certain oil-lubricated plastic bearings was due to a fluid-trapping pocket being formed in the plastic.
Early water-lubricated rubber bearings were not very effective because the bearings did not develop hydrodynamic lubrication. Water-lubricated bearings in the 1960's were in the category of medium wear devices, the same category as automotive brake linings. This was because the rubber was too thick, the surface was too rough and the side edges were rounded.
Thin rubber bearings have different wear and friction action from harder nonmetallic materials used in bearings. When a radial load is initially applied to a thin rubber bearing, the deflection is elastic and disappears if the load is removed. After a short period of time under load, the deformation becomes permanent and remains after load removal because of the compression set present in every rubber compound. This action is called Plasto-Elastohydrodynamic lubrication. This type of favorable lubrication action is enhanced by the special elastic-creep properties of some rubber compounds. It was found that thin nitrile rubber bearings for rotating shafts or journals partially immersed in water developed a continuous, unbroken film of water resulting from hydrodynamic lubrication. Tests of rotating journals on thin rubber bearings formed a plastic-elastohydrodynamic pocket in the rubber. It was determined that the pressure developed by the hydrodynamic pumping process deformed the rubber, pushing it out towards the sides and ends of the bearing contact patch forming a non-contact region or pocket in the center of the contact patch rubber, i.e. plasto-elastohydrodynamic (“PEH”) lubrication. It was further found that bearing sample patches made of polymeric plastic materials harder than rubber initially deflected less than samples made of thin rubber, but eventually developed deep grooves and wore away after partially completing the 28-day test procedure.
There have been developed brass backed rubber staves since the 1920's, and plastic backed rubber staves since 1976. The prior art further includes rubber in compression impinging on slippery surfaces such as Teflon, ultra high molecular weight, polyethylene (UHMWPE) and other materials. See U.S. Pat. Nos. 3,743,306, 4,568,056, 4,626,112 and 4,737,688.